This invention relates to hydrocarbon conversion processes using an iron oxide catalyst and more particularly relates to extending iron oxide-containing catalyst life in alkyl aromatic dehydrogenation processes.
Iron oxide catalysts have been used widely in various conversion processes including dehydrogenation and hydrocarbon synthesis. Examples of these processes include dehydrogenation such as ethylbenzene to styrene, ethyltoluene to vinyltoluene and butenes to butadiene; formation of gasoline fraction hydrocarbons from synthesis gas (hydrogen and carbon monoxide) as in the Fischer-Tropsch process; dealkylation of alkylaromatics such as toluene to benzene and synthesis of ammonia from nitrogen and hydrogen As in most heterogeneous catalytic systems, an iron oxide catalyst typically declines in conversion activity over time. A process that could extend the useful life of an iron oxide catalyst would be very advantageous. The process of this invention contacts molecular oxygen over an iron oxide catalyst at very low concentrations which surprisingly prolongs catalyst life. The use of oxygen in hydrocarbon conversion processes with an iron oxide catalyst has been reported in several U.S. patents. However, the concentrations of oxygen used therein are either substantially greater than the effective range demonstrated in this invention or are used in conjunction with an oxidation catalyst to provide process heat. Generally, use of oxygen in such patents was for a different purpose than prolonging catalyst life as discovered in this invention.
U.S. Pat. No. 3,437,703 describes a process to dehydrogenate ethylbenzene to styrene over a catalyst containing an iron oxide component and an oxidation catalyst component in the presence of oxygen (0.01 to 10 mole percent of total process stream). The oxygen is used in conjunction with the oxidation catalyst (typically salts of platinum and palladium) to "burn" hydrogen formed in the dehydrogenation process to provide heat necessary to drive the endothermic dehydrogenation reaction and to remove gaseous hydrogen from the reaction system which thermodynamically favors styrene production. U.S. Pat. No. 3,855,330 likewise describes adding oxygen to an ethylbenzene dehydrogenation process using an oxidation catalyst together with an iron oxide dehydrogenation catalyst. Again the oxygen is used to oxidize hydrogen or carbon-containing materials to provide process heat. This patent discloses oxidizing and dehydrogenating in separate zones and teaches that, although oxygen is present in the oxidation zone, less than 0.01 mole of oxygen per mole of ethylbenzene is permitted in the dehydrogenation zone and preferably there is a practically complete absence of oxygen in the gases leaving the oxidation zone.
U.S. Pat. No. 3,502,737 discloses incorporating oxygen in an ethylbenzene dehydrogenation process at levels of 0.01 to 1.0 moles of oxygen per mole of ethylbenzene to burn carbon and hydrogen to produce process heat which reduces steam requirements. The data contained in this patent show that styrene yield generally declines as the oxygen content increases.
U.S. Pat. No. 2,945,900 discloses using oxygen at 5 to 40 vol.% levels in a dehydrogenation process using calcium nickel phosphorus-type atalyst. Processes using oxygen with an iron oxide catalyst are excluded specifically in this patent.
U.S. Pat. No. 4,039,601 describes a method to regenerate a coked iron oxide catalyst with oxygen and steam by continuously removing catalyst from a reactor, regenerating such removed catalyst outside the reactor and returning the regenerated catalyst to the reactor.
Another process using oxygen in an ethylbenzene dehydrogenation reacts about 0.5 mole of oxygen together with steam and an inert gas with ethylbenzene to form styrene and water over various catalysts including iron oxide. Such oxidative dehydrogenation operates at low temperature and typically results in poor yield with substantial carbon oxide formation.
U.S. Pat. No. 3,505,422 describes adding carbon dioxide to a feedstream of steam in a dehydrogenation process using an iron oxide catalyst to reduce the amounts of undesirable by-products.